1. Field of the Invention
The present invention relates to a high-frequency filter using a resonator having a transmission line in the form of a coplanar line, and more particularly to a high-frequency filter in which an attenuation pole is provided in transmission characteristics of the filter.
2. Description of the Related Art
A high-frequency filter used in a significantly high frequency band (generally, 1 to 100 GHz) such as of microwave bands and millimeter wave bands is widely used as a functional device indispensable for transmission/reception apparatuses in various radio communication facilities, fiber-optic high-speed transmission apparatuses, and measuring instrument related to the above apparatuses. In recent years, high-frequency filters having a microwave integrated circuit structure have been also used as high-frequency filters used in the significantly high frequency band for with ease in promoting the scale down. For example, in U.S. Pat. No. 6,798,319, disclosed is a high-frequency filter using a resonator having a transmission line in the form of a coplanar line. The transmission line in the form of the coplanar line is a transmission line of a coplanar structure made from a metal conductor in which a high-frequency transmission line is disposed on one main surface of the substrate. The resonator having the transmission line in the form of the coplanar line is called a coplanar line resonator.
FIG. 1A is a plan view showing an example of a conventional high-frequency filter using a coplanar line resonator. FIG. 1B is a cross-sectional view taken along line B-B in FIG. 1A.
Ground conductor 2 is disposed on one main surface of substrate 1 made of a dielectric material. Rectangular opening 3 is formed in ground conductor 2. In opening 3, center conductor 4, which functions as signal line disposed on the one main surface of substrate 1, is provided so as to extend in the longitudinal direction of opening 3. A coplanar line resonator is configure by ground conductor 2 disposed on one main surface of substrate 1 and center conductor (i.e., signal line) 4 inside opening 3 formed in ground conductor 2.
Center conductor 4 is provided with an electric length depending on a dielectric constant of substrate 1 in accordance with a desired resonance frequency (center frequency f0) of the filter. Usually, when a wavelength corresponding to center frequency f0 is λ, the electric length of center conductor 4 is set to λ/2. In other words, the electric length of the coplanar line resonator is set to λ/2 relative to center frequency f0. Both ends of center conductor 4 are spaced apart from ground conductor 2 at both ends (the right and left ends as shown) of opening 3, thereby forming electrically open ends. This arrangement allows generation of a standing wave having a null point of the voltage displacement at a midpoint bisecting center conductor 4 in the longitudinal direction and maximum voltage displacements of mutually reverse polarities at both ends, as indicated by curve S in FIG. 1B, acting as a resonator. The coplanar line is an unbalanced transmission line in which a high-frequency wave travels caused by electric field E generated between center conductor 4 and ground conductor 2 and by magnetic field induced by the electric field. In FIG. 1A, electric field E is indicated by arrows.
Input line 5a and output line 5b are mounted on the a other main surface of substrate 1 at positions respectively corresponding to the ends of center conductor 4. Input line 5a and output line 5b are input/output signal lines made of microstrip lines which are electromagnetically coupled with one end and the other end of the coplanar line resonator, respectively. Input line 5a is arranged as a linear transmission line extending from a left end (as shown) and is overlapped with one end side (i.e., input side) of center conductor 4 through substrate 1 so as to be electromagnetically coupled. On the other hand, output line 5b includes a closed loop line portion surrounding the right end (i.e., output end) as shown of the coplanar line resonator and a linear extension portion extending from the closed loop line portion to the right end (as shown) of substrate 1. The closed loop line portion of output line 5b is formed in an approximate rectangle and extends transversely across center conductor 4 near the right end thereof. A position where the top portion, namely, the closed loop line portion transverses center conductor 4 through substrate 1 is defined as transverse portion X. In FIGS. 1A and 1B, transverse portion X is positioned at the output end side rather than the midpoint of center conductor 4.
In this arrangement, by the electric field and the magnetic field generated at the input end side of the coplanar line resonator and generated between center conductor 4 and ground conductor 2, input line 5a electromagnetically couples with the resonator. By the electric field and the magnetic field generated at the both end sides of the transverse portion X in output line 5b and generated between center conductor 4 and ground conductor 2, output line 5b electromagnetically couples with the resonator. As shown, electric field components are indicated by arrows. With this electromagnetic coupling, high-frequency components propagating to the coplanar line resonator from input line 5a are filtered by the coplanar line resonator, and filtered high-frequency components are obtained in output line 5b. When the position of transverse portion X is close to the output end of center conductor 4, as a high-frequency filter, it is possible to obtain a band characteristic (resonance characteristic) of a single peak characteristic in which center frequency f0 is regarded as the center, as indicated by curve A in FIG. 2.
Now, since the closed loop line portion of output line 5b transverses center conductor 4, another boundary condition is generated in the coplanar line resonator. Transverse portion X in the closed loop line portion is overlapped with center conductor 4 to be electrically coupled. Since this coupling is capacitive coupling, in view of transverse portion X, this coupling is equivalent to that microstrip lines are respectively connected to input/output end sides of center conductor 4. Therefore, for example, the output side of center conductor 4, namely, an electrical open end of the coplanar line resonator is provided with an electric length based on distance d1 to the output end as the microstrip line. When considerations are given to frequency f1 in which distance d1 is set as one-quarter wavelength, this microstrip line functions as an electrical short-circuited end for frequency f1.
FIG. 3 shows an equivalent circuit of the above-mentioned high-frequency filter. When an input point of input line 5a is represented by Vin and an output point of output line 5b is represented by Vout, first resonance circuit Zf0 by the coplanar line resonator is serially connected as a serial arm between input terminal Vin and output terminal Vout, and second resonance circuit Zf1 generated by the fact that the closed loop line portion crosses center conductor 4 at transverse portion X is connected as a parallel arm between the output side of first resonance circuit Zf0 and a ground potential point. In FIG. 3, both resonance circuits are represented by LCR serial circuits. A frequency at a serial arm resonance point by first resonance circuit Zf0, namely, a resonance frequency is f0, and a frequency at a parallel arm resonance point by second resonance circuit Zf1 is f1. In second resonance circuit Zf1, the current is maximized at frequency f1 in which distance d1 between transverse portion X and the output side end of center conductor 4 is one-quarter wavelength. Consequently, as indicated by curve C in FIG. 2, attenuation pole P occurs in a band characteristic as the high-frequency filter.
In this arrangement, since transverse portion X is positioned at the output end side rather than the midpoint of center conductor 4, distance d1 between transverse portion X and the output side end of center conductor 4 is shorter than λ/4 when a wavelength corresponding to center frequency f0 is λ. As a result, parallel arm resonance point f1 by distance d1 is higher than the serial arm resonance point, namely, center frequency f0. Attenuation pole P by parallel arm resonance point f1 is formed in the higher-frequency range than center frequency f0 in the band characteristic of the high-frequency filter, makes an attenuation gradient of the band characteristic steeper and makes a passband width, in which, for example, the attenuation amount is in a range from the passband peak value to 3 dB, narrows. Therefore, an apparent Q value of the high-frequency filter is increased.
On the other hand, at one side (i.e., input end side) of center conductor 4, since center conductor 4 electromagnetically couples with input line 5a, the input end side of center conductor 4 viewed from transverse portion X is not electrically short-circuited end. When a distance from transverse portion X to the input end of center conductor 4 is represented by d2, d2>λ/4 is satisfied. When a frequency in which distance d2 is one-quarter wavelength is represented by f2, ripple P′ regarding f2 as the parallel arm resonance point generates in the band characteristic of the high-frequency filter, however, is inadequate to form attenuation pole P distinctly at f2.
The above description relates to the case in which transverse portion X is positioned between the midpoint and the output end of center conductor 4. When transverse portion X is positioned between the midpoint and the input end of center conductor 4, namely, when distance d1 is longer than λ/4, an attenuation pole by parallel arm resonance point f1 occurs in the lower-frequency range than center frequency f0.
According to the above description, in the high-frequency filter, output line 5b is formed in the closed loop line while input line 5a is linearly formed. However, an input line may be formed in a closed loop line and an output line may be formed in a linear transmission line, and an attenuation pole is also formed in this case similarly to the above description. Further, both of an input line and an output line may be formed in closed loop lines. When both of the input line and the output line are formed in closed loop lines, a distance between a transverse portion in each closed loop line portion and a corresponding end in the center conductor is shorter than the half length of the center conductor, and therefore each attenuation pole by each closed loop line portion occurs in the higher-frequency range than center frequency f0.
In the conventional high-frequency filter using the single coplanar line resonator as described above, attenuation pole P generated by the closed loop line portion in the output line is principally formed in either one of the higher-frequency range or the lower-frequency range with respect to center frequency f0 of the coplanar line resonator. As a result, it is difficult to make the passband width of the high-frequency filter narrow and to increase the Q value by arranging attenuating poles at both of the high-frequency range and the low-frequency range around center frequency f0. Also, when the attenuation pole is formed, as shown in FIG. 2, the attenuation amount increases by α(>0) at center frequency f0 of the passband compared with a case where no attenuation pole is formed, and there is a problem in which an insertion loss increases correspondingly.